Opioids are widely used in patients with advanced cancers and other terminal diseases to lessen suffering. Opioids are narcotic medications that activate opioid receptors located in the central nervous system to relieve pain. Opioids, however, also react with receptors outside of the central nervous system, resulting in side effects including constipation, nausea, vomiting, urinary retention, and severe itching. Most notable are the effects in the gastrointestinal tract (GI) where opioids inhibit gastric emptying and propulsive motor activity of the intestine, thereby decreasing the rate of intestinal transit and producing constipation. The effectiveness of opioids for pain is often limited due to resultant side effects, which can be debilitating and often cause patients to cease use of opioid analgesics.
In addition to analgesic opioid induced side effects, studies have suggested that endogenous opioid compounds and receptors may also affect activity of the gastrointestinal (GI) tract and may be involved in normal regulation of intestinal motility and mucosal transport of fluids in both animals and man. (Koch, T. R. et al, Digestive Diseases and Sciences 1991, 36, 712-728; Schuller, A. G. P., et al., Society of Neurosoience Abstracts 1998, 24, 524, Reisine, T., and Pasternak, G., Goodman & Gilman's The Pharmacological Basis of Therapeutics Ninth Edition 1996, 521-555 and Bagnol, D., et al., Regul. Pept. 1993, 47, 259-273). Thus, an abnormal physiological level of endogenous compounds and/or receptor activity may lead to bowel dysfunction.
For example, patients who have undergone surgical procedures, especially surgery of the abdomen, often suffer from a particular bowel dysfunction, called post-operative (or post-surgical) ileus, that may be caused by fluctuations in natural opioid levels. Similarly, women who have recently given birth commonly suffer from post-partum ileus, which is thought to be caused by similar natural opioid fluctuations as a result of birthing stress. Gastrointestinal dysfunction associated with post-operative or post partum ileus can typically last for 3 to 5 days, with some severe cases lasting more than a week. Administration of opioid analgesics to a patient after surgery, which is now an almost universal practice, may exacerbate bowel dysfunction, thereby delaying recovery of normal bowel function, prolonging hospital stays, and increasing medical care costs.
Opioid receptor antagonists such as naloxone, naltrexone, and nalmefene, have been studied as a means of antagonizing undesirable peripheral effects of opioids. However, these agents act not only on peripheral opioid receptors, but also on central nervous system sites, so that they sometimes reverse the beneficial analgesic effects of opioids, or cause symptoms of opioid withdrawal. Preferable approaches for use in controlling opioid-induced side effects include administration of peripheral opioid receptor antagonist compounds that do not readily cross the blood-brain barrier. For example, the peripheral μ opioid receptor antagonist compound methylnaltrexone and related compounds have been disclosed for use in curbing opioid-induced side effects in patients (e.g., constipation, pruritus, nausea, and/or vomiting). See, e.g., U.S. Pat. Nos. 5,972,954, 5,102,887, 4,861,781, and 4,719,215; and Yuan, C. -S. et al. Drug and Alcohol Dependence 1998, 52, 161. Similarly, peripherally selective piperidine-N-alkylcarboxylate and 3,4-dimethyl-4-aryl-piperidine opioid receptor antagonists have been described as being useful for treatment of opioid-induced side effects constipation, nausea or vomiting, as well as irritable bowel syndrome and idiopathic constipation. See, e.g., U.S. Pat. Nos. 5,250,542, 5,434,171, 5,159,081, and 5,270,328.
It would be desirable to provide peripheral μ opioid receptor antagonist compounds for administration to a patient in need of treatment for any of the above-mentioned disorders.